This invention relates to an information signal transmitting device responsive to plural information signal transmission requests for plural requesting parties for incidentally transmitting information signals. More particularly, it relates to an information signal transmitting device for transmitting the information signal from its leading end after lapse of a shorter waiting time since acceptance of a request for transmission of information signals.
For transmitting picture signals responsive to plural incidental picture signal transmission requests from plural requesting parties, it may be contemplated to lay in store dedicated reproducing units, such as video tape recorders R.sub.1, R.sub.2, . . . R.sub.n for individual requesting parties H.sub.1, H.sub.2, . . . H.sub.n, as shown in FIG. 1, and to supply picture signals to monitoring devices M.sub.1, M.sub.2, . . . M.sub.n of the individual requesting parties. If, with such method, the requesting party H.sub.1 has requested to view a group or file T.sub.n of picture signals, referred to herein as a file, in place of a file T.sub.1 the requester is currently viewing, it is necessary for the transmitter or the purveyor to exchange in a video tape recorder R.sub.1 a video tape V.sub.1 having the file T.sub.1 recorded therein for a video tape V.sub.n having the file T.sub.n recorded therein. Consequently, some waiting time is produced since the requesting party H.sub.1 has made a request until the desired pictures are actually purveyed.
On the other hand, if the requests by a requesting party H.sub.2 and by another requesting party H.sub.n are directed to the same file, such as a file T.sub.2, it is necessary for the purveyor to lay in store plural video tapes V.sub.2 each having the file T.sub.2 duplicated thereon.
Thus a system may be envisioned in which, instead of providing video tape recorders for individual requesting parties, a disc-shaped recording medium 100, having compressed pictures stored therein, as shown in FIG. 2, is laid in store for complying with the requests by the requesting parties for transmission of picture signals. The disc-shaped recording medium 1 has stored therein data of compressed pictures. The compressed picture data is read out from the recording medium 1 in time multiplexing. The compressed picture data read out in time multiplexing is expanded by a time axis expander 2 so as to be supplied to expanders 3.sub.1, 3.sub.2, . . . 3.sub.n provided at the requesting parties H.sub.1, H.sub.2, . . . H.sub.n. The picture information expanded by the expanders 3.sub.1, 3.sub.2, . . . 3.sub.n is transmitted to the monitoring devices M.sub.1, M.sub.2, . . . M.sub.n.
Since the readout of the compressed picture data from the disc-shaped recording medium 1 is carried out time-divisionally, the picture data compressed on the time axis per unit time is sent out to plural requesting parties. The disc-shaped recording medium 1 may be read out while the picture information of plural files is random accessed per unit time.
If there are n requesting parties requesting transmission of pictures for an interval of t seconds, picture data for the time of nt may be read out by random accessing.
Thus, with a system employing the disc-shaped recording medium, the problem inherent in the above-mentioned system employing the video tape recorder may be resolved to some extent.
For example, the labor of exchanging the video tapes on request of file exchange may be dispensed with, while it becomes unnecessary to duplicate video tapes of the same file in readiness for a situation wherein plural requests are made for the same file.
Meanwhile, in the system employing the above-mentioned disc-shaped recording medium, the picture information cannot be purveyed instantaneously to the requesting parties, even although the operation of duplication, as required with the system employing the video tape recorder in case plural requests from different requesting parties are directed to the same file, is not required.
The reason is that, by reading out the pictures of each requested picture file by large blocks with a view to reducing the effects of access time losses of the disc-shaped recording medium, the sequence-waiting time for the requesting parties is protracted.
For reading out information data from the disc-shaped recording medium, the access time, which is the sum of the seek time necessary for the readout head to be moved as far as the information data storage position and the waiting time which elapses until stabilization of rotation of the disc-shaped recording medium, is required.
If the access time is rendered constant, and the data readout block size is changed, the data readout efficiency is changed as shown in FIG. 3. The data readout efficiency is inherently the efficiency based on the data transfer capacity per second. In FIG. 3, the access time is set to 0.045 second and the maximum readout speed is set to 30 Mbps. In FIG. 3, the readout block size is plotted on the abscissa. If the readout block size is 0.5 second, for example, the data readout efficiency is based on continuous data readout for 0.5 second.
In FIG. 3, if the readout block size is 0.5 second, the transfer efficiency plotted on the ordinate is 0.917 (91.7%). The reason is that, since the readout block size is only 0.5 second, the number of times of accessing needs to be increased despite the transfer capacity being inherently 30 Mbps. If the readout block size is 0.1 second, the transfer efficiency is lowered to 0.690 (69%).
If the maximum readout speed, the readout block size and the access time are set to R, B and Ta, respectively, the readout transfer efficiency RA is represented by EQU RA=R.times.(B/(B+Ta))
Consequently, the readout block size needs to be increased for improving the transfer efficiency.
However, if the readout block size is increased for raising the transfer efficiency, the sequence-waiting time is protracted when the requesting party has raised a request for a new file, so that the waiting time until the pictures of the file become available for the user is protracted, as a result of which the requesting party cannot view the pictures instantaneously.